It has long been recognized that many of the waste products generated by human society can, ultimately, be broken down into a small number of simple organic materials that have their own intrinsic value. If this transformation could be achieved in an energy-efficient manner, and on a large enough scale, then there could be enormous benefits to society.
Most living materials, as well as most synthetic organic substances used in domestic and commercial applications comprise carbon-based polymers of various compositions. Under appropriate conditions, most such materials—including wood, coal, plastics, tires, and animal waste—will break down to a mixture of gaseous products, oils, and carbon. Materials such as agricultural waste products may also contain inorganic substances that break down to mineral products. Almost all of these products, whether organic or inorganic, can enjoy new lives in a host of beneficial and often lucrative applications.
Not only is the principle of creating useful materials from otherwise unserviceable waste appealing: recycling of waste materials is of fundamental importance to the way that the burgeoning human population will come to cope with major challenges in the 21st century. Two principal challenges facing humanity are coping with a finite supply of materials and energy, and with curtailing the growing threat to the environment from global warming. Indeed, an idea that is rapidly gaining currency is that recycling carbon-based materials from within the biosphere rather than introducing new sources of carbon from underground oil, natural gas and coal deposits could mitigate global warming.
As of today, however, industries that produce huge volumes of waste products comprising largely organic materials face enormous challenges in disposing and storing that waste, as well as putting it to maximum beneficial use.
A case in point, the food processing industry around the world generates billions of pounds of organically rich wastes per year. These wastes are associated with the processing of both animal and plant products, and include turkey-, fish-, chicken-, pig-, and cattle-processing and husbandry wastes. The food processing industry continues to grow and its members face significant economic and environmental pressures to do something productive with their waste products. Such waste products give rise to a number of critical problems. The generation of greenhouse gases such as carbon dioxide and methane by landfilling, land applying, or digesting food wastes, without any other benefit, is one such problem. Ideally, the food industry must adopt efficient and economical ways of managing their wastes without discharging odorous or objectionable pollutants.
More recently, the cost of warehousing unusable byproducts in many areas is growing in significance. As the types of waste products that can be fed to agricultural livestock become increasingly regulated. For example, in the wake of BSE/CJD scares in Europe, many waste products are simply being warehoused, pending a suitable fate. Clearly, there is an additional urgent need to find an acceptable means to cleanly process and utilize such materials. Preferably, a way to convert food-processing wastes into useful, high-value products needs to be found.
An additional drive to seek treatment alternatives is the combined enforcement of wastewater discharge regulations and the escalation of sewage surcharges. The food processing industry must seek cost-effective technologies to provide pretreatment or complete treatment of their wastewaters and solid (wet) wastes. Historically, food processing facilities located within or adjacent to municipalities, have relied on local publicly owned treatment works (POTWs) for wastewater treatment and disposal. Increasingly, this option is becoming less available, as a result of more rigorous enforcement. Pressure to comply with wastewater discharge permits has increased. Dwindling federal grants for construction of new and upgraded POTWs also mean that this option is less appealing. Thus, the food-processing industry is increasingly being pressured with regard to how to effectively dispose of its inedible products.
Bioaccumulation of persistent chemicals such as dioxins and the potential for the spread of life threatening diseases such as Mad Cow Disease (BSE) is another threat to food processors and food consumers alike. This threat is greatly exacerbated by refeeding food processing residues to farm animals. Food processors need economical solutions to break this cycle.
Furthermore, municipal and regional sewer authorities are requiring industries to reduce their organic biochemical oxygen demand (BOD), chemical oxygen demand (COD), and solid loading on the sewers. Due to the high BOD concentrations typically found in high-strength food process wastewaters with high levels of suspended solids, ammonia, and protein compounds, the food processing industry is under additional scrutiny. Food processing facilities need cost-effective and application-specific treatment technologies to manage their wastewaters and solid wastes effectively.
Similar problems are multiplied, magnified and augmented in many different ways across other industries. For example, the generation of malodorous air emissions associated with rendering plants—that convert animal waste by heat into fats and proteins, is one such problem. Another is land application of municipal biosolids that contain high concentrations of pathogens.
There have been various approaches developed to process used waste tires, say from truck and passenger vehicles, into useful products including fuels, petroleum oils, carbon, fuel-gases, and feedstocks for manufacture of tires and other rubber products. Typically, these schemes involve heating and dissolving the tires in solvents. Some of the schemes attempt to devulcanize the tire rubber, i.e., break the sulfur bonds that connect the constituent polymers along their lengths. Others attempt to depolymerize the rubber material. Depolymerization breaks the long chain polymers into shorter ones that are more fluid so can more easily be used as a product such as a fuel oil. Some schemes involve the use of water under conditions near or above its critical point (˜3,200 psi and˜370° C.) where water is a very good solvent for, and reactant with, the tire material. However, such schemes are energetically inefficient because of the energy required to achieve super-critical conditions. Furthermore, processing at super-critical conditions also requires expensive super-alloy operating equipment.
A number of organic materials have also been investigated for dissolving tire material to form a heavy oil or a devulcanized rubber product. Generally, the existing schemes that operate at modest conditions (<200 psi) produce heavy, contaminated products, whereas those that use lighter solvents produce better products but must use a solvent that is expensive, or that requires high pressure (<2,000 psi), or both. Additionally, most schemes that use a solvent to dissolve tire material are uneconomical because some fraction of the solvent is lost during the process and there is a cost associated with the make-up solvent, even in instances where solvent recovery and reuse can be practiced.
Aerobic and anaerobic digesters have been employed at sewage treatment plants to treat municipal sewage sludge. There are a number of problems associated with their use. The basic principle behind their operation is that biologically rich materials are directed into large holding vessels that contain bacteria which digest the biological materials. Typically, dissolved solids are directed to an aerobic digester, and suspended solids are directed to an anaerobic digester. Once the nutritional feed materials are exhausted, the bugs can no longer sustain themselves, and they die. The end-product of the digestion period is a sludge that contains the dead bacteria, and which must be disposed of in some way. One problem with the resulting material is that it still contains pathogens. Problems with the whole process, in general, include that the holding times in the digester vessels can be as long as 17 days, and that the operating conditions are difficult to maintain. For example, the relatively large vessel (typically 20–30 ft. in diameter) is usually maintained at above 85° F., and in some cases above 122° F.
All of the disposal technologies currently available to industries, in particular the food processing industry, have significant limitations and drawbacks that provide an incentive to search for alternative processes. This applies to technologies in addition to the use of existing POTWs. In particular, four types of approach, land disposal (landfills, composting, land application), biotreatment, traditional thermal oxidation treatments such as incineration/combustion, and pyrolysis/gasification, all have separate drawbacks.
Drawbacks for land disposal include: high haulage or transport costs, significant potential for groundwater contamination from leaching, and the exposure of area residents to high concentrations of hazardous pollutants (such as pathogens in the instance of land application). Landfills produce gas that can create air pollution concerns, including the generation of greenhouse gases.
Disadvantages for biotreatment of waste include difficulty with control, and inability to verify performance because of the difficulty with verifying adequate airflow into the soil. The airflow must be maintained to provide oxygen if using aerobic bacteria. For example, bacteria that may have been developed to consume specific compounds will, when placed in soil, activate alternative enzyme systems to consume the easiest available compounds.
Drawbacks with older units that carry out incineration or combustion include the requirement to add equipment to meet air pollution emission standards that are continually being made more stringent by the government. It may also take longer to obtain air discharge permits for incinerators than for other technologies due to significant community concerns about incineration. Additionally, the treatment of the waste at the exhaust means treating large volumes of gas so that very large plant equipment is required. The feedstock is also low in calorific value. Some incinerators are not compatible with solid fuels or solid waste, as these materials will start to oxidize too high up in the furnace. Conversely, high moisture content in the feedstocks is also a problem because during incineration or combustion the water is vaporized and removed—a process which requires approximately 1,000 Btu/lb of water vaporized. This represents huge heat/energy losses to the system.
The last category of technique employed—pyrolysis/gasification—is appealing because, unlike the others mentioned, it attempts to convert the waste into utilizable materials, such as oils and carbon. Of principal concern when searching for optimum ways of breaking down waste products is how to adjust the composition of the resulting materials while minimizing the amount of energy needed to effect the breakdown. In the past, the principal pyrolysis and gasification methods that have been employed attempted to break down the waste products in a single stage process, but a single stage has been found to offer inadequate control over purity and composition of the end products.
Pyrolyzers have been used to break down organic materials to gas, oils and tar, and carbonaceous materials. A pyrolyzer permits heating of the organic materials to high temperatures, ˜400–500 ° C., but has poor energy efficiency and gives little control over the composition of the resulting materials. In particular, most waste products—especially those from the agricultural industry—contain up to 50% water. The pyrolyzer needs to boil off that water, a process that is very energetically demanding. Additionally, a pyrolysis chamber tends to be large in order to maximize throughput, but then gives rise to significant temperature gradients across the chamber. Thus, the pyrolysis process involves an uneven heating of the waste products and leads to poor quality or impure tars and oils in the resulting end products.
Gasifiers have been used to achieve a partial combustion of waste products. In essence, a gas—usually air, oxygen, or steam—is passed over the waste products in an amount that is insufficient to oxidize all the combustible material. Thus, some combustion products such as CO2,H2O,CO,H2 and light hydrocarbons are produced, and the generated heat converts the remaining waste products into oils, gases, and carbonaceous material. The gases produced will contain some of the input gases, but any gases that are produced are too voluminous to be stored and must be used immediately or piped to a place where they can be utilized. Gasifiers also suffer from some of the same drawbacks as pyrolyzers: for example, a water-containing waste product will consume a lot of energy in vaporizing the water content.
Both pyrolysis and gasification methods additionally have the problem that the resulting materials contain unacceptable levels of impurities. In particular, sulfur—and chlorine-containing materials in the waste products give rise, respectively, to sulfur-containing compounds such as mercaptans, and organic chlorides in the resulting end products. Typically, chlorinated hydrocarbons at levels of 1–2 ppm can be tolerated in hydrocarbon oils, but neither gasification nor pyrolysis methods can guarantee such a low level with any reliability.
Furthermore, pyrolysis and gasification methods have low efficiencies, typically around 30%. One reason for this is that the products are not optimum in terms of calorific content. A further reason is that, in a single stage process, the materials are not produced in a form that easily permits their energy to be use fully re-used within the process. For example, it is difficult to capture the thermal energy in the solid products that are produced and redirect it to assist in the heating of the reaction vessel.
Overall, then, pyrolysis/gasification methods suffer in several ways. The oil product is generally rich in undesirable high viscosity components such as tar and asphalt. Both pyrolysis and gasification processes have poor heat transfer properties and consequently do not heat evenly. Therefore, end products vary greatly in number with few of sufficient quantity or quality for economical recovery. Wet feedstocks require significant energy to vaporize and represent large energy losses to the system since the water leaves as a gas in the stack. Thus, in summary, the disadvantages of pyrolysis/gasification are that the overall operating cost is high, the process is capital intensive and some by-products may have limited or no value.
Although there have been many variants of the pyrolysis and gasification methods, all of which have suffered from broadly similar drawbacks, one recent advance has permitted significant increases in processing efficiency. For example, U.S. Pat. Nos. 5,269,947, 5,360,553, and 5,543,061, disclose systems that replace the single-stage process of the prior methods with a two-stage process. In a first stage (often referred to as the “wet” stage), the waste products are subjected to heat at around 200–250° C. and at about 20–120 atmospheres pressure. In a preferred embodiment, the waste products are subjected to a pressure of about 50 atmospheres. Under such conditions the water content of the waste material hydrolyzes many of the biopolymers such as fats and proteins that may be present to form a mixture of oils. In a second stage (often called the “dry” stage), the mixture is flashed down to low pressure, during which around half of the water is driven off as steam. The mixture is heated still further to evaporate off the remaining water while the mixture ultimately breaks down into gaseous products, oils, and carbon.
The principal advance of these two-stage methods was to permit generation of higher quality and more useful mixtures of oils than any of the previous single stage processes. However, the products of such methods still suffer from problems of contamination, from materials such as sulfur—and chlorine-containing compounds, and the need to evaporate a significant portion of the water still entails a substantial energy penalty. Thus, prior two stage methods have been difficult to make commercially viable.
Accordingly, there is a need for a method of processing waste and low-value products to produce useful materials in reliable purities and compositions, at acceptable capital and operational cost.